1. Field of the Invention
The present invention relates to an electronic throttle control apparatus that serves to detect an open-circuit failure of a motor in a throttle actuator for controlling the amount or degree of opening of a throttle valve arranged in an intake pipe of an automotive engine.
2. Description of the Related Art
In a throttle control apparatus for detecting an open-circuit failure of a motor energization system in an throttle actuator, a motor for driving a throttle valve is controlled in a PWM (pulse width modulation) manner, and a motor current is controlled by changing a drive duty ratio in the PWM control, so that the degree of opening of the throttle valve is adjusted to a target opening degree. In such a throttle control apparatus, when the drive duty ratio is higher than a predetermined value, and when the current flowing through the motor is less than a predetermined value, it is determined that an open circuit occurs in the motor (see, for example, a first patent document: Japanese patent No. 3,189,717).
However, it is necessary to detect the current flowing through the motor so as to find an open-circuit of the motor, so there is a problem that a current detection circuit is required, thus adding to the cost of manufacture thereof.
In addition, in case where the throttle actuator is driven to operate by controlling the motor current through PWM driving, an electric time constant of the motor used in the throttle actuator is generally small, and hence the pulsation (ripple) of the motor current generated due to the on/off control of the energization of a motor winding at the time of PWM driving is large, so the detected value of the motor current will also vary greatly depending upon current detection timing in a PWM drive period, thus posing another problem that it becomes difficult to properly make the setting of a current value used for the determination of detection of a motor open circuit.
Moreover, in case where a shunt resistor for detection of the motor current is provided so that a motor current signal flowing through the shunt resistor is input as a voltage signal to an AD converter and is AD converted at predetermined timing synchronized with the above-mentioned PWM drive period so as to detect the motor current, there is a further problem that the processing load of the microcomputer is increased.
Further, in case where the motor current signal is to be smoothed for current detection by means of a smoothing circuit for the purpose of reducing the processing load of the microcomputer, there arises a still further problem of generation of a detection delay.